1. Technical Field of the Invention
The present invention relates to medical magnetic resonance imaging (MRI) for visualizing the inner body of an object to be diagnosed on a magnetic resonance phenomenon, in particular, to MR imaging capable of depicting such fluid as lymphatic vessels and lymph nodes which not only flow at slower speeds but also are thinner than blood vessels.
In this specification, imaging for lymphatic vessels and lymph nodes in accordance with the present invention is referred to as xe2x80x9cMR Lymphography.xe2x80x9d
2. Description of Prior Art
Magnetic resonance imaging is based on a technique that magnetically excites nuclear spins of an object placed in a static magnetic field with an RF signal of a Larmor frequency thereof, acquires an MR signal emanated therefrom due to the excitation, and reconstructs an image using the MR signal.
In the field of MR imaging, there is provided MR angiography (MRA) for imaging blood vessels. At present, this MRA is accomplished by various techniques including a time of flight (TOF) method, a phase contrast (PC) method and a method of enhanced MR angiography with contrast agent.
As to thoracic duct flowing also in the body, no imaging method has been provided so far in the field of MRI, although lymph liquid appears to be similar to blood, due to the fact that the lymph liquid flows at slower speeds than blood through thinner ducts than blood vessels.
Conventionally, lymphatic vessels of an object are visualized on the X-ray Lymphography with contrast agent injected into the object. It is further reported that lymph nodes can be visualized by performing MR imaging with contrast agent injected into an object.
However, any conventional technique of imaging lymphatic vessels and lymph nodes requires injection of contrast agent. In general, it takes a long time (e.g., a few hours) to inject such contrast agent, which imposes heavy mental and physical burdens on a patient. It is frequently true that contrast agent is characteristically undesirable for a patient, resulting in imaging using contrast agent that can not provide a universal solution to lymphography. Thus, at present, it is the case that there is no imaging method permitting an easy diagnosis of lymphatic-system diseases.
The present invention has been performed in consideration of the drawbacks that the foregoing conventional imaging for lymphography faces. An object of the present invention is to easily depict lymph liquid that flows through lymphatic vessels and lymph nodes with no contrast agent.
In order to realize the above object, in an MRI system according to the present invention, by scanning means, a pulse sequence of an effective echo time (TEeff) selected to depict desired fluid other than blood within an object is performed and an MR signal generated in response to the performance of the pulse sequence is acquired. And by producing means, an image wherein the fluid is highlighted is produced from the MR signal.
Thus, because the effective echo time is selected as a time that agrees with depiction of fluids (such as lymph liquid or liquid residing in recesses) with respect to the length of T1 time of blood, an MR image highlighting the fluid can be provided without using any contrast agent. This MR image is greatly useful to understand the flow and behavior of the liquid such as lymph liquid, making it useful to diagnose lymph system diseases.
It is preferred that the scanning means performs the pulse sequence toward the fluid which is made up of lymph liquid existing in a lymphatic vessel, which flows at a slower speed than the blood. It is also preferred that the effective echo time is selected to be long with respect to the T1 (spin-lattice relaxation) time (normally, 900 to 1000 msec) of the blood.
For example, the effective echo time is selected to be a relatively long time such that both the lymphatic vessel and a blood vessel existing around the lymph liquid are depicted together, the lymph vessel having priority over the blood vessel in contrast. By way of example, the pulse sequence is either a two-dimensional sequence or a three-dimensional sequence. It is possible that the pulse sequence is based on one of FSE (fast SE) method, FASE (Fast Asymmetric SE) method, EPI (Echo Planar Imaging) and fast recovery (FR) methods. The pulse sequence may be based on an FASE (Fast Asymmetric SE) method wherein echo train spacing (ETS) is selected to be a relatively short time. Preferably, the effective echo time TEeff is selected from a range of 120 to 800 msec and the echo train spacing ETS is selected from a range of 3 to 10 msec.
It is preferred that the pulse sequence is a three-dimensional sequence and the producing means produce three-dimensional image data using the echo signal and obtain the image by performing maximum intensity processing with the three-dimensional image data.
Preferably, the above main MR system configuration further comprises detecting means for detecting a signal indicative of a cardiac temporal phase of the object, the scanning means including scan start means for repetitively starting the pulse sequence synchronously with time delayed by a certain interval from a heartbeat-specific reference wave appearing in the signal. For instance, the certain interval delayed by the scan start means is selected from a range of 500 to 600 msec so as to acquire the echo signal at a diastole of a heart of the object.
In addition, the above main MR system configuration may further comprise instructing means for instructing the object to hold their breath during the trip a pulse sequence is performed by the scanning means. For example, the breath holding may be instructed based on an intermittent breath-holding method.
Still further, in the above main MR system, the fluid may be liquid filled in a concavity within the object.
On the other hand, an MR imaging method of the present invention comprises the steps of: not merely performing a pulse sequence of echo time (TEeff) selected to depict desired fluid other than blood within an object but also acquiring an MR signal generated in response to the performance of the pulse sequence, and producing from the MR signal an image wherein the fluid is highlighted. Thus, an MR image highlighting the fluid can be provided without using any contrast agent.